Nullability Annotations to Java Classes

[ParaskP7]

This is the parent issue for the Add Nullability Annotations to Java Classes for JP/WPAndroid & FluxC project.

Platform Request: N/A Project Thread: paaHJt-5fa-p2

This project is about adding missing nullability annotations (@Nullable & @NonNull) to as many Java-related classes as possible, including fields, arguments, parameters, and return values.

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### Tasks
- [ ] #2803
- [ ] #2798
- [ ] #2799
- [ ] #2800
- [ ] #2853
- [ ] #2827
- [ ] #2801
- [ ] #2802
- [ ] #2854

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### Stretch Goals
- [ ] #2818
- [ ] #2822